There are a couple of different options. One would be to have more material added on top of the texture and then troweled to a smoother finish. This could add a lot of weight and “could” cause some adhesion problems. The other choice is to have the texture sanded smooth. That’s a lot of dusty work. Whichever method you choose, make sure the walls are properly primed before applying a topcoat so you get a uniform appearance.